The production of polyolefins using single-site transition metal catalysts is rapidly becoming dominant in the industry. The advantages of these catalysts over traditional Ziegler catalysts are primarily that the molecular weight dispersity is relatively narrow, and for polyethylene, that long-chain branching can be obtained. In the case of propylene, styrene, and other higher olefins, the tacticity of the polymers can be controlled, leading to varying amounts of crystallinity. The polymers produced from single-site catalysts are therefore highly valued in the marketplace.
For many applications, such as blending, dyeing, and improving paintability, it would be desirable to incorporate polar comonomers to improve the wetability and compatibility of the surface. Polar comonomers which contain heteroatoms with at least one hydrogen are especially desirable, as the possibilities for hydrogen bonding and nucleophilic reactions to improve adhesion are increased. A drawback of all single-site transition metal catalysts as well as heterogeneous Ziegler catalysts is that they are notoriously sensitive to non-hydrocarbons, especially those with heteroatom-hydrogen groups. Nevertheless, the incorporation of polar comonomers into polyolefins using such catalysts has been reported. For example, Iwata, et al in DE 1,947,590 (1970, assigned to Mitsui Petrochemical Industries Ltd.) used a heterogeneous Ziegler catalyst to copolymerize hindered phenols with olefins. The phenol was pretreated with an equivalent amount of triethyl aluminum (or Et.sub.2 AlCl) prior to exposure to the catalyst. This forms an aluminum complex which prevents poisoning of the catalyst.
Single-site transition metal catalysts have also been used to incorporate polar comonomers into polyolefins. Wilen and Nasman (Macromolecules 1994, 27, 4051-4057) describe the copolymerization of propylene with alkenyl substituted phenols using a zirconocene dichloride catalyst. Aluminum cocatalysts, methylalumoxane (MAO) or trimethyl aluminum, were used to activate the zirconocene dichloride and to form an aluminum complex of the phenol. The necessity of preforming an aluminum complex is shown in the following quote: "The low activity at low Al/phenol ratios (&lt;4.4) may be attributed to rapid catalyst deactivation in the absence of an excess of free MAO or TMA (Me.sub.3 Al)." In another publication by Wilen et al (Polymer 1992, 33(23), 5049-5055) on a similar topic, it is concluded: " . . . it is necessary to pretreat the functional monomer with a protecting group in order to prevent catalyst poisoning during polymerization. Catalyst poisoning was verified by experimental propylene polymerization runs under standard experimental conditions conducted in the presence of 2,6-di-t-butylphenol. As may be anticipated, even a small amount of 2,6-di-t-butylphenol was capable of deactivating most of the Ziegler-Natta polymerization sites as presented in Table 1. On the contrary, when the 2,6-di-t-butylphenol was pretreated with a stoichiometric amount of triethylaluminium to liberate ethane and generate alkylaluminium phenoxide, no severe deactivation of polymerization sites was detected."
The requirement for the use of aluminum reagents to prevent catalyst deactivation has disadvantages which have prevented the commercial application of this technology. Most importantly, for most applications the relatively large amounts aluminum in the copolymer product must be neutralized and removed. This requires that the copolymer be dissolved and treated with acid, a process which adds extra steps and considerable cost to the process. Also, the cost of the aluminum reagents is significant.
In light of these problems, it would be desirable to have a process for incorporation of polar comonomers which contain heteroatom-hydrogen groups into polyolefins without the need for stoichiometric amounts of cocatalyst.